The invention relates to a photometric device for determining the gross calorific value of a test gas with a radiation source that generates a measuring beam and a spectral unit for dividing the measuring beam spectrally, with a test cell for the absorption of the test gas and a radiation receiver, which generates electric measuring signals in dependence on the measuring beam intensity and which is electrically connected with an evaluation unit that is equipped with at least one signal amplifier for amplification of the measuring signals, being arranged successively in the path of the measuring beam.
The invention furthermore relates to a method for the photometric determination of the gross calorific value of a test gas, in which a test cell that is filled with test gas is interspersed with the measuring beam, in which the measuring beam intensity that is permitted to penetrate by the test cell is measured with the help of a radiation receiver that generates measuring signals, in which the measuring signals are generated in relation to appropriate measuring signals without test gas in the test cell and spectral absorption values that are allocated to wave ranges are generated, and in which the allocated spectral absorption values are amplified with the help of at least one signal amplifier.
From U.S. Pat. No. 4,594,510 we know of a photometric device and a method for determining the gross calorific value of a test gas that is equipped both with a radiation source that generates a measuring beam and a spectral unit for dividing the measuring beam spectrally. A test cell for the intake of the test gas and a radiation receiver are arranged in the path of the measuring beam. With the radiation receiver, electric measuring signals can be generated in dependence on the measuring beam""s intensity. The radiation receiver is electrically connected with an evaluation unit, which is equipped with at least one signal amplifier for the amplification of the measuring signals. In order to determine the gross calorific value of test gases whose material composition is not known, a calibration must first be performed during which the gross calorific values of calibrating gases, which contains at least as many material components as the test gas of unknown composition, are determined. Since the exact material composition of the test gas is inherently not known, in practice the calibration is generally performed with considerably more material components in the calibrating gases than would theoretically be required. For sufficient accuracy of the determination of the gross calorific value, it is furthermore necessary that when determining the proportionality factors for amplifying the measuring signals the spectral support areas are selected under the assumption of a certain, very likely accurate material composition of the test gas in order to record the material components adequately. In practice, it showed that sufficiently exact determinations of gross calorific values of test gases with different origins, and therefore with different material compositions, are very complex.
From DE-A-48 00 279 we know, with regard to the methods and devices for the determination of physical properties of samples in the short-range infrared spectral region, that a Fourier transform spectrometer is used.
In the report by H. M. Heise entitled xe2x80x9cInfrarotspektrometrische Gasanalysexe2x80x9d (Infrared Spectrometric Gas Analysis) from the publication xe2x80x9cInfrarotspektroskopiexe2x80x9d (Infrared Spectroscopy), issued by H. Gxc3xcunzler, Springer-Verlag, Heidelberg, Germany 1996, both dispersive and non-dispersive methods and spectrometers are revealed for performing these methods. While a dispersive device is equipped with a dispersive element such as a grating for the spatial splitting of infrared radiation in dependence on its wavelength, the revealed non-dispersive devices are, for example, equipped with a filter device for selecting a wavelength.
The gross calorific value of a gas creates a connection between the gas volume consumed during combustion and the amount of heat produced during the same and has achieved great importance for the control engineering of natural gas operated equipment, for example. Among gas mixtures such as natural gas, the gross calorific value is dependent upon the composition of the gas mixture. When purchasing natural gas, the gas volume is generally used as the basis for calculating the purchase price, with the gas gross calorific value affecting the purchase deal directly. Therefore, a volume-related purchase price assumes information about the gross calorific value of the gas intended to be purchased in order to justify a higher purchase price for gases with high gross calorific value over less expensive offers.
Due to the ongoing liberalization of the energy market, which is associated with the decline of regional energy monopolies, natural gases of various providers and composition will be fed through joint pipeline systems in the near future. The quality of the natural gas used by an end user from the pipeline system is therefore not known until it is consumed so that problems with regard to volume-related purchase price billing can occur. It would, therefore, be desirable to have a method and a device that would allow a determination process of the gross calorific value of a gas, for example when removing it from the pipeline system, that is quick and requires few metrology efforts. This way, calculating the cost on the amount of heat achieved with the gas is possible without the provider knowing the gas that is used or its exact composition.
Direct measurement of the gross calorific value of a gas is generally performed with calorimeters. For this, a specified volume of the test gas is burned, and the thermal energy released thereupon to a defined quantity of a coolant medium is measured by the temperature increase of the coolant medium. Suitable coolant media are for, example, air or water. While the high degree of inertia of the measurement proves disadvantageous for the quick recording of the gross calorific value of a natural gas, despite its high degree of accuracy when utilizing water, the disadvantages for utilizing air as a coolant medium lie especially in the complicated mechanics for setting a certain quantitative proportion of gas, combustion air and cooling air.
We furthermore know of cost-intensive calorimeters based on stoichiometric combustion, where a certain air requirement that is needed for the combustion of a defined quantity of the test gas is determined.
One method for determining the gross calorific value indirectly involves gas chromatography, in which the gas composition is determined quantitatively and the gross calorific value of the overall gas mixture is calculated based on knowledge of the gross calorific values of the individual components. The disadvantages of gas chromatography are the high procurement costs of the necessary devices as well as the personnel qualifications required for their operation.
Compared to gas chromatography, previously-known methods for determining the gross calorific value place fewer demands on the types of devices required to perform the method and have the benefit of a shorter measuring time, especially in comparison with the calorimetric method. For this method, an absorption spectrum of the natural gas is measured in the short-range or medium-range infrared spectral region, which is composed cumulatively of the sum of the individual spectra of the gas components present in the gas, and analyzed with the help of suitable spectral analysis methods. The percentage of absorbance of a component from the overall spectrum thus determined is equal to the concentration percentage of this component in the test gas. Thereafter, based on knowledge of the respective gross calorific value of the components, the gross calorific value of the entire gas mixture is calculated. The spectral analysis, however, is difficult due to heavy overlapping of the absorption bands of different gas components, often leads to inaccurate results and requires a very high degree of computation.
The invention is therefore based on the task of further developing a device and a method of the kind described above in such a way that the gross calorific value, even of a test gas with an unknown material composition, can be determined quickly, simply, inexpensively and reliably.
In a device in accordance with the invention the task is resolved by being able to determinexe2x80x94from the composition of the test gas with regard to the components or functional groups and the position of absorption bands and absorption linesxe2x80x94independent partial spectral regions of an absorption band and/or a spectral region comprising absorption lines, by being able to amplifyxe2x80x94with the or every signal amplifier that is allocated to the partial spectral regionxe2x80x94the measuring signal coming from this partial spectral region with a degree of amplification that is allocated to this partial spectral region, and by the fact that the evaluation unit comprises a summer for summing up the amplified measuring signals.
In the method, the task is resolved in accordance with the invention by being able to determinexe2x80x94from the composition of the test gas with regard to the components or functional groups and the position of absorption bands and absorption linesxe2x80x94independent partial spectral regions of an absorption band and/or a spectral region comprising absorption lines, by being able to amplifyxe2x80x94with the or every signal amplifier that is allocated to the partial spectral regionxe2x80x94the measuring signal coming from this partial spectral region with a degree of amplification that is allocated to this partial spectral region, and by the fact that the amplified measuring signals are added for the calculation of the gross calorific value of the test gas.
The reaction heat generated during gas combustion is based on the oxidation of Cxe2x80x94H bonds, with the thermal quantity that is generated being dependent upon the respective bond energy. The invention is based on the idea that the oscillations of the Cxe2x80x94H bonds, which have among each other the same defined bonding energy and generate the same amount of heat during combustion, interact with electromagnetic radiation at an allocated wavelength. Based on this prerequisite, the gross calorific value of the gas can be calculated from wavelength-dissolved measurements in fixed partial spectral regions independent on the material composition of the test gases and from a weighting of the integral spectral absorption values of these partial spectral regions. The allocation of the Cxe2x80x94H oscillations to a defined previously-known component of the gas is therefore only required within the framework of additional corrective procedures at best so that an often inaccurate, comprehensive spectral analysis can be avoided.
In a useful development of the invented device, the evaluation unit is an electric circuit, and for the purpose of adjusting the degrees of amplification one or more adjustable control voltage sources, whose respective electric voltage controls the amplification degree of the allocated signal amplifier, are incorporated.
In one variant version, the evaluation unit is a digital computing device that is equipped with a sum memory, with means of adjustment with an amplification parameter memory for storing amplification parameters that are allocated to the respective partial spectral regions being incorporated for adjusting the amplification degrees, with the signal amplification being conducted by multiplying the measuring signalsxe2x80x94through a central processing unit of the computing devicexe2x80x94with the amplification parameters in dependence on the spectral property of the measuring beam generating the measuring signal.
In an advantageous version, the photometric device is equipped with a dispersive element as the spectral unit for the purpose of spatial splitting of the measuring beam in dependence on its wavelength and also with a detector row as the radiation receiver with detector elements that are arranged next to each other, with each detector element being connected to a signal amplifier, respectively. In such a design where the device is further developed, the dispersive element is a diffraction grating.
In a variant further development, the invented photometric device comprises a spectral switch unit for selecting the partial spectral regions of the measuring beam and a detector element as radiation receiver, with means of adjustment, which are coupled with the spectral switch unit and with which the amplification degrees can be adjusted, being incorporated. In such a development, the spectral switch unit is a filter wheel with spectral filters, triggered by a filter wheel drive.
In another useful further development, the spectral unit is an interferometer, which is equipped with a beam splitter for splitting the measuring beam into two optical paths, which are limited by a stationary mirror or a mirror that can be moved with a control element. The mirrors are aligned in such a way that the portions of the measuring beam reflected from them unite in a joint beam path. Beyond that, the control element is coupled with means of adjustment, with which the amplification degrees can be set.
In a preferred version, the radiation source is an infrared radiation source generating infrared radiation in the medium-range infrared spectral region and the radiation receiver is a sensitive radiation receiver in the medium-range infrared spectral region.
In a version deviating from this one, the radiation source is an infrared radiation source generating infrared radiation in the short-range infrared spectral region and the radiation receiver is a sensitive radiation receiver in the short-range infrared spectral region.
In a useful further development, the invented device is equipped with a spectral accumulator, with which the measuring signals can be stored in pairs with the respectively allocated partial spectral regions.
In a preferred design of the invented method, the measuring beam is split spatially in dependence on its wavelength.
In a version deviating from this one, the invented method involves the modulation of the amplitude of the measuring beam with the help of an interferometer.
In a beneficial version, the spectral absorption values are amplified with allocated partial spectral regions from the spectral region of the Cxe2x80x94H oscillation, and particularly from the range between 3 xcexcm and 4 xcexcm.
In a version deviating from this one, the spectral absorption values are amplified with allocated partial spectral regions from the spectral region of the Cxe2x80x94H harmonic oscillation, and particularly from the range between 1.5 xcexcm and 2 xcexcm.
It is useful to check the test gas for at least one interfering foreign gas, whose spectral absorption values are known in dependence on the wavelength and which does not contribute to the gross calorific value, by measuring characteristic spectral absorption values with established partial spectral regions, to determine the percentage of the foreign gas in the test gas in dependence on the characteristic spectral absorption values and to subtract the percentage of the foreign gas interfering with the measurement from the measured spectral absorption values.
In a beneficial version, the test gas is examined for special gases contributing to the gross calorific value by measuring characteristic spectral absorption values at established wavelengths; these gases differ from a main component of the test gas based on their chemical composition or structure, such as due to additional functional groups or branching of hydrocarbon chains.
In a beneficial version, the partial spectral regions of allocated spectral absorption values are stored as a spectrum in the spectral accumulator.